Furnace blower speed control

ABSTRACT

A furnace blower speed control system adapted to vary the speed of a furnace blower with respect to time, the system including solid state means energizable to effect a delayed start of the blower at a low speed after a predetermined time following furnace burner ignition and to gradually increase the speed of the blower as the furnace temperature rises until full blower speed is reached. The system is also operable to maintain the full blower speed for a predetermined time after the burner is extinguished and thereafter gradually decrease the speed of the blower as the furnace cools down.

United States Patent Bauer et a1.

[451 Oct. 14, 1975 FURNACE BLOWER SPEED CONTROL Inventors: Frederick T. Bauer; Ronald E.

Holkeboer, both of Holland, Mich.

Assignee: kobertshaw Controls Company,

Richmond, Va.

Filed: Dec. 7, 1973 Appl. No.: 422,729

US. Cl 236/11; 318/334 Int. Cl. F24D 5/00 Field of Search 236/11, 9; 318/11, 331,

References Cited UNITED STATES PATENTS 2/1968 Moreland 236/11 X 1/1970 Moreland 236/11 X Primary Examin'erEdward G. Favors v Attorney, Agent, or Firm-Malcolm R. McKinnon 57 ABSTRACT v A furnace blower speed control system adapted to vary the speed of a furnace blower with respect to time, the system including solid state means energizable to effect a delayed start of the blower at a low speed after a predetermined time following furnace burner ignition and to gradually increase the speed of the blower as the furnace temperature rises until full blower speed isnreached. The system is also operable to maintain the full blower speed for a predetermined time after the burner is extinguished and thereafter gradually decrease the speed of the blower as the furnace cools down.

14 Claims, 8 Drawing Figures US. Patent Oct. 14, 1975 Sheet10f2 3,912,162

US. Patent Oct. 14, 1975 Sheet20f2 3,912,162

E5 n n N A ,FURNACE BLOWER SPEED CONTROL BRIEFISUMMARY OF THE iNvEN'noN in the plenum or bonnet of the furnace. Such systems tend'to blow cold air from the heating ducts out of the heating registers at a speed fast enough to cause discomfort to the occupants, and such systems are also subject to the defect that the speed of the blower changes in discrete steps which are noticeable and annoying to the occupants of the building.

Other prior blower speed control systems utilize a heavy bimetallic type sensor mounted in the furnace bonnet or plenum with a push-pull-cable operating a variable speed pulley through a troublesome linkage system which is difficult to adjust, service and maintain. Still other prior blower speed control systems utilize a thermistor type heat sensor mounted in the furnace plenum or bonnet. However, the proper placement of the thermistor type heat sensor requires extensive investigation' to ascertain the correct location for the best response to temperature changes within the fumace and such types of systems are also defective in that the output produces a nonsymmetrical voltage wave form causing the blower motor to run noisily and also hot, thereby wasting electrical energy. This last mentioned type of control is also subject to a practical problem with the thermocouple type gas furnace controls in that they give off enough heat in the summer to raise the bonnet temperature to the start temperature of the control thereby causing the blower to start with the result that the occupants believe the furnace is on in the summer. Moreover, controls utilizing thermistor type sensors are subject to the defect that if the thermistor is not carefully and properly matched with the system, the control willcause the blower motor to hunt or oscillate slowly up and down in speed with resultant inefficiency.

An object of the present invention is to overcome the aforementioned as well as other disadvantages in prior furnace blower speed control systems of the indicated character, and to provide an improved furnace blower speed control system incorporating improved means for starting, stopping and varying the speed of a furnace blower as a function of time.

Another object of the invention is to provide an improved fumace blower speed control system incorporating solid state means energizable to effect a delayed start of a furnace blower at a low speed following furnace burner ignition; to gradually increase the speed of the blower as the fumace temperature rises until full blower speed'is reached; to maintain full blower speed for a predetermined time after the furnace burner is extinguished; and thereafter to gradually decrease the speed of the blower to the stopped condition as the furnace .cools down.

Another object of the invention is to provide an improved fumace blower speed control system that may be utilized with standard shaded pole and/or permanent split capacitor type motors of the type which are standard throughout the industry.

Another object of the invention is'to provide an improved blower speed control system which eliminates the necessity of utilizing two or three speed temperature sensing switches, thermistor type sensing switches or variable speed pulley arrangements in the furance bonnet or plenum.

Another object of the invention is to provide an improved fumace blower speed control system which utilizes low voltage control wiring for the control circuitry thereof. L

Another object of the invention is to provide an improved fumace blower speed control system which may be mounted on or near a fiirnace without regard to position or sensing location and the calibration of which is not affected by changes in line voltage.

Another object of the invention is to provide an improved fumace blower speed control system which may be utilized with either gas or oil fired furnaces.

Another object of the invention is to provide an improved fumace blower speed control system which starts the furnace blower at a speed so low that the noise when the blower starts generally cannot be heard by the building occupants; which accelerates the blower to full speed slowly so that the noise is hardly noticeable by the building occupants; which, by starting at a slow speed, drives the cold air out of the ducts at a very low velocity so that the cold air emanating from the air registers is hardly noticeable; and which, when the room air space is warm enough and the thermostat open, will decrease the speed of the blower infinitely and slowly so as to provide less Stratification of the heat in the room.

Another object of the invention is to provide an improved fumace blower speed control system which may be programmed for heat exchangers with different heating and cooling characteristics during manufacture of the system and which may be modified in the field to suit the various requirements of the heating system characteristics and/or the customer.

Yet another object of the present invention is to provide an improved furnace blower speed control system which will not turn on in the summer due to high heat in the plenum chamber of gas fired furnaces with thermocouple type controls, which will not cause the blower motor to hunt or oscillate slowly up and down in speed as with the thermistor control systems; and which shuts off all power to the motor at the end of a heating cycle thereby preventing heating of the blower motor and the wasting of electrical energy between heating cycles. v

. The above as well as other objects and advantages of the present invention will become apparent from the following description, the appended claims and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic circuit diagram of a furnace blower speed control system embodying the present invention;

FIG. 2 illustrates the line voltage as supplied to the control system and blower motor at full speed;

FIG. 3 illustrates the voltage supplied to the control circuit illustrated in FIG. 1;

FIG. 4 illustrates the voltage at the capacitor C9 with the motor off;

FIG. 5 illustrates the voltage at the capacitor C9 with the motor running slowly;

FIG. 6 illustrates the voltage to the blower motor running slowly;

FIG. 7 illustrates the voltage at the capacitor C9 with the blower motor at approximately 90 percent full speed; and

FIG. 8 illustrates the voltage at the blower motor at approximately 90 percent full speed.

DETAILED DESCRIPTION Referring to the drawings, and more particularly to FIG. 1 thereof, the circuitry for a furnace blower speed control system, generally designated 40, embodying the present invention is schematically illustrated therein. As shown in FIG. 1, the system 40 is comprised of a step down power transformer T1 having a primary winding 42 and a secondary winding 44, the primary winding 42 being adapted to be connected to a conventional source of 120 volt alternating current. The system 40 also includes a conventional thermostat 46, a burner control 48 which may be of any conventional type operable in response to the thermostat to control the furnace burner (not shown) and deenergize the furnace burner in the event an unsafe condition occurs. The system 40 also includes a blower motor 50 which may be a standard shaded pole motor or a permanent split capacitor type, both of which are standard throughout the heating industry.

In accordance with the present invention, the system v40 includes a thermostat interface transformer T2 having a primary winding 52 and secondary windings 54 and 56; resistors R1 through R9, R11 through R16, R19 through R27 and R29 through R31; potentiometers R10, R17, R18 and R28; capacitors C1 through C9; a Zener diode D1; power diodes D2 through D5; and signal diodes D6 through D15. In addition, the system 40 includes transistors Ql through Q4, and Q7; programmable unijunction transistors Q5 and Q6; a silicon controlled rectifier Q8; and a triac Q9. A fuse F1 and a choke Ll are also provided as illustrated in FIG. 1, the above described components all being electrically connected by suitable conductors as illustrated in FIG. 1.

Referring in greater detail to the components and unique sub-circuits illustrated in FIG. 1, the programmed Zener diode Q1 is used to adjust the up speed rate of the blower motor 50. The Zener diode Q1 is used to maintain a constant voltage difference across the resistor R12 to charge up the timing capacitor C4. In operation, a forward bias is applied to the base of the diode O1 to cause it to conduct. This bias is adjusted by the resistor R12. A constant voltage across the diode Q1 emitter-collector will tend to maintain a constant bias to the base thereof to maintain the constant emitter-collector voltage. The resistor R14 is utilized for negative feedback to reduce the base bias as the emitter voltage (measured from common) increases due to the capacitor C4 charging. The base drive to the diode Ql must be reduced as the emitter voltage increases due to the fact-that as current through the diode Q1 decreases, the gain increases.

The system 40 includes, a Start-Stop control circuit which is used to give toggle or hysteresis to the startstop points. It will be appreciated that without hysteresis, any control would have the tendency to stutter at the start and stop points. The programmable unijunction transistors OS and Q6 are embodied in the start-stop hysteresis control circuit. The transistor Q4 functions as an input control amplifier and phase inverter. With the control in the off state, the transistor O4 is not conducting, the collector of the transistor Q4 and the gate of the programmable unijunction transistor Q5 are preferably at +13 volts (collector supply voltage). The gate of the programmable unijunction transistor Q6 is at some voltage less than 13 volts as set by the potentiometer R28. As the capacitor C9 is charging up to 13 volts through the resistor R27, the anode voltage of the programmable unijunction transistor Q6 will be equal to its gate voltage and turn on before the programmable unijunction transistor O5 is able to turn on. (The Q5 gate is at 13 volts.)

When'the programmable unijunction transistor Q6 turns on, the positive pulse that appears across the resistor R30 charges the capacitor C8 positive through the signal diode D12 to hold the transistor Q4 off more. When the control comes on, the transistor Q4 starts to conduct. When the collector voltage of the transistor Q4 and gate of the programmable unijunction transistor Q5 are lower than the gate voltage of the programmable unijunction transistor Q6, the transistor Q5 will turn on, inhibiting Q6. This will stop the positive pulse across the resistor R30. The capacitor C8 will discharge to allow the transistor Q4 to conduct more, turning the transistor Q5 on sooner, to give hysteresis for start.

Hysteresis for stopping operates in an opposite manner. When the control is going off, (slowing down) the collector of the transistor Q4 and the gate of the transistor Q5 are increasing in voltage. When they reach a voltage greater than the gate of Q6, Q6 will turn on to charge the capacitor C8, to turn the transistor Q4 off more, to insure that the programmable unijunction transistor Q5 does not turn on again. The start speed is adjusted by the setting of the potentiometer R28. The stop speed is preferably set 10 to 30% higher than the start speed by the value of the resistors R29 and R31. For example, the system is preferably adjusted so that the motor 50 starts at approximately 200 250 R.P.M. and stops at 300 350 R.P.M. The low voltage interface is provided by the current transformer T2. When an AC. current of 0.2 to 1.2 amps. is passed through the primary winding 52 a voltage is produced in the secondary. This is rectified and filtered to turn the transistor Q7 on.

The triac turn on circuit uses the silicon controlled rectifier Q8 to pull the gate of the triac Q9 negative for phase control of the power to the blower motor 50. A silicon controlled rectifier is used because it has memory when applied with direct current or pulsating direct current. In other words, it will remain on, once it is turned on until the anode current is removed. This memory characteristic is required when the blower motor 50 is running at full speed. It is at this time that the triac Q9 will receive a tum-on pulse before it has turned off. If the gate of the triac O9 is not held on through zero current crossing, the triac Q9 will turn off on the next half wave and cause noise and heating of the motor 50 and waste of electrical power. It will be appreciated that up and down speed rate control is normally set during manufacturing but also can be adjusted in the field to suit the homeowner and/or heating system characteristics.

Typical values for the components in thesystem 40 illustrated in FIG. 1 are as follows:

Resistors: R1 1K OHM V2 WATT R2 2.7 OHM 10% A WATT R3 II II I! R4 470 I I II I I R5 150 5 WATI' R6 82 k WATT R7 270 II II II R8 39K II II II R9 1.5K A WA'IT R10 2.5 MEG. POT R11 1 MEG 10% IWATT R12 470K R13 39K II II II R14 27K II II II R15 100K II II II R16 10K OHM 10% WATT R17 10K POT m8 II II II R19 10% A WATT R20 II II I II II R21 3.9K OHM 10% V2 WATT R22 33K 5 1 WATT R23 3.9K WATT R24 3 3K I I II I I R25 1.5K V4 WATT R26 150 A WATT R27 56K R28 50K POT R29 100K 10% k WATT R30 47 II II II R3 470K II II II Capacitors:

Cl 0.25 MFD 500V. DISC CERAMIC C2 .47 400V. MYLAR FILM C3 .025 500V. DISC CERAMIC I C4 100 20V. TANTALUM CS 4.7 10V. TANI'ALUM C6 4] II II II C7 .02 150V. DISC CERAMIC C8 .22 100V. MYLAR FILM C9 .047 100V. MYLAR FILM Diodes:

D1 ZENER DIODE 13V. 1 WATT D2 through 5 POWER DIODE D6 through 15 SIGNAL DIODE Semiconductors:

Q1 through 04 TRANSISTOR Q5 and Q6 P.U.T. v Q7 TRANSISTOR Q8 SILICON CONTROLLED RECTIFIER Q9 TRIAC Miscellaneous:

F1 10A. 150V. FUSE Ll 70MICRO- CHOKE HENRY In further explanation of the time base control portion of the system 40, the up-down rate switch is the interface between the low voltage thermostat wiring and the blower speed control. As shown in FIG. 1, the input transformer (T2) primary is wired in series with the thermostat low voltage wires (the heating section if heating and cooling provisions are both provided). When the primary winding 52 of the transformer T2 has from 0.2 to 1.2 amperes of thermostat current in it, the voltage developed in the secondary is rectified by the diodes D8 and D9. This voltage is filtered by the capacitor C5 and applied to the base of the transistor Q7 through the current limiting resistor R7 to forward bias the transistor Q7 into conduction. Diodes D6 and D7 are used to limit the maximum voltage to 1.4 volts. The resistor R6 is for negative feedback to limit the base current to a safe value. When the transistor Q7 conducts, its emitter is pulled up to collector supply voltage. This supplies voltage to the up-down rate switch, triac phase control and forward biases the diode D10 to put an initial voltage on the capacitor C4. The up rate adjustment is used to adjust the charging current into the capacitor C4. This controls the rate at which the blower motor 50 increases in speed or the time for the blower motor to reach full speed. Such adjustment is preferably set during calibration and is field adjustable to suit the home owner and/or heating system characteristics.

As shown in FIG. 1, the components for the up rate adjust circuit are the resistors R12, 13, 14, 15, 16, the rate adjustment potentiometer R17, the transistor Q1 and the signal diode D11. The transistor O1 is utilized as a programmable Zener in that its emittercollector voltage can be set by the potentiometer R17. The transistor O1 is used to maintain a constant voltage above (more positive) the voltage at the emitter of the transistor Q2, which is approximately 1.2 volts below (less positive) the voltage on the timing capacitor C4. This will give a constant voltage across the resistor R12 and signal diode D11 to charge the capacitor C4 at a constant current or rate for a linear change in power to the blower motor 50. The down rate adjustment is used to adjust the discharging current from the timing capacitor C4 when the thermostat 46 opens. This controls the rate at which the blower motor 50 decreases in speed or the time for the blower motor to stop after the thermostat opens, the down rate adjustment also preferably being set during calibration, but is field adjustable to suit the home owner and/or heating system characteristics. As illustrated in FIG. 1, the components forthe down rate adjustment circuit are the potentiometer R10, the resistor R1 1, the signal diode D14 and the signal diode D15. The signal diode D14 is used to clamp the voltage at the collector of the transistor Q1 approximately 0.6 volts below (less positive) the voltage at the emitter of the transistor Q3. This will give a constant voltage for the potentiometer R10 and resistor R11 to discharge into to give a constant current from the timing capacitor C4 for a linear change (decrease) in power to the blower motor. It will be appreciated that the up-down control is a time to voltage converter. Its output is a DC voltage proportional with time. The output is used to drive the triac firing circuit.

The components of the up-down control circuit are the transistors Q2 and Q3, the capacitor C4, C6, the signal 7 diodes D10 and 13, and and time to start adjustable potentiometer R18, and voltage divider, resistors R20 and 21. The diode D13 charges the capacitor C6 to a level determined by the voltage divider R20-2l, to provide a DC voltage source for the Darlington current amplifier comprising transistors Q2 and Q3.

When the up-down rate switch transistor O7 is turned on, the signal diode D10 charges the capacitor C4 to an initial voltage level, to overcome the base emitter drops of the transistors Q2 and Q3 and voltage is produced across the potentiometer R18 by the current through the up-rate adjustment circuit. The up rate adjustment then charges the capacitor C4 up at a linear rate. The Darlington amplifier follows this change in voltage to produce a voltage proportional to time. This output voltage is taken from the arm of potentiometer R18.

The voltage at the top of the potentiometer R18 is used as the reference voltage point for up and down rate adjustment switches.

Continuing the description of the operation of the system 40, the triac phase control circuit is used to control the turn-on of the triac Q9. The power to the blower motor 50 is somewhat proportional to voltage at the input to the phase control circuit (base of the transistor Q4). Inaccordance with the present invention, this section also has some hysteresis built into it to prevent anfindeterminate condition where the triac Q9 might not come on for the next cycle and cause heating of the motor 50 and wasting of power. It thus overcomes any unstable initial triggering at large phase angles. The triac phase control circuit also has the start speed adjustment potentiometer R28 and the circuit that stops the motor at a higher speed than when it started. The components of this section are the transistor Q4, the programmable unijunction transistors Q5 and Q6, the silicon controlled rectifier Q8, the capacitors C7, C8 and C9 and the resistors R19, and R22 through R31.

The triac Q9 functions as an AC Switch that is turned on by the triac phase control circuit. The triac Q9 is turned on late in the AC voltage cycle for low speed, earlier in the voltage cycle for faster speed and on all the time for full speed. The triac Q9 is turned on when the programmable unijunction transistor Q5 turns on, producing a positive pulse across the resistor R26 to turn on the SCR Q8, which in turn pulls the triac Q9 gate negative. If the motor 50 is running slow and the triac Q9 is turning on late in the voltage cycle, the triac Q9 and SCR Q8 turn on simultaneously with the pulse at the resistor R26. If the motor 50 is running at maximum speed, the pulse at the resistor R26 will occur before the triac current has crossed through zero motor current. In this condition, the SCRS will turn on to pull the gate negative. When motor current goes through zero, the triac Q9 will turn off and back on im mediately because the gate was pulled negative some time before. This feature is important in controlling inductive loads.

The power supply preferably uses a 17 volt stepdown transformer T1 with a full wave bridge rectifier. A 13 volt Zener diode is also preferably used to regulate the low voltage to the control circuit. This voltage is not filtered but is pulsating DC. This is to synchronize the turn on pulse for the triac Q9 to line voltage to maintain good speed regulation and symmetrical voltage wave shape applied to the blower motor 50. It will be appreciated that symmetrical waveshape prevents motor overheating.

When the line voltage has been applied and the thermostat has been open long enough for the blower motor 50 to be ofi, the control is said to be off. In the off condition, the power transformer T1 is supplying the control with regulated pulsating DC voltage. The capacitor C9 is charging through the resistor. R27. When the capacitor C9 reaches the voltage level of the gate of the programmable unijunction transistor Q6, as set by potentiometer R28, Q6 will turn on to discharge the capacitor C9, (see FIG. 1). The positive pulse appearing across the resistor R30 will charge the capacitor C8 positive. A portion of this voltage is applied to the emitter of the transistor Q4 to hold it off and will hold the programmable unijunction transistor Q5 off to inhibit the triac Q9. When the thermostat closes the transformer T2 will turn on the transistor Q7 to activate the up rate adjust circuit to start charging the capacitor C4. This will also forward bias the diode D10 to put an initial charge on the capacitor C4. As the capacitor C4 continues to charge, the emitter of the transistor Q3 will follow (approximately 1.2 V. below) the voltage level of the capacitor C4. The positive going voltage at the wiper of the potentiometer R18 is applied to the base of the transistor Q4 through the resistor R19. As this level increases the transistor Q4 will begin to conduct when the base voltage is above the emitter bias supplied by the capacitor C8. As the transistor Q4 conducts more, the collector voltage will become less positive. As the capacitor C4 continues to increase in voltage, the transister Q4 will increase in conduction to reduce its collector voltage. This will cause the programmable unijunction transistor O5 to turn on earlier in the cycle to cause the triac Q9 to turn on earlier in the cycle to apply more power to the blower motor 50. As full speed is reached, the triac turn-on impulse from Q5 through the resistor R26 will occur before the motor has gone through zero current. The silicon controlled rectifier Q8 will be turned on by the pulse from the transistor Q5 to the silicon controlled rectifier O8 to hold the triac Q9 on again as soon as it turns off due to motor current going through zero. In the embodiment of the invention illustrated, the timing capacitor C4 will continue to charge past the full speed point to let the blower motor 50 run at full speed for about l minute after the thermostat 46 opens. The amount of over-charge is limited by the voltage on the filter capacitor C6. When the voltage on the capacitor C4 is approximately equal to the voltage of the collector of the transistor Q2, the base-collector junction becomes forward biased and limits the voltage that the capacitor C4 can charge to. When the thermostat 46 opens, the transistor Q7 shuts off. The resistors R6, R8, and R9 pull the emitter of the transistor Q7 to common (0. volts). This shuts off the up rate adjust and activates the down rate adjustment circuit comprising the potentiometer R10, resistor R1 1, and signal diodes D14 and D15. When the capacitor C4 discharges to the stop speed, the collector voltage of the transistor Q4 is higher than the voltage at the arm of the potentiometer R28, and the programmable unijunction transistor Q6 will turn on. When Q6 turns on, the positive pulse that is developed across the resistor R30 charges the capacitor C8 positive. Part of this positive charge is applied to the emitter of the transistor Q4 to turn it off more than to insure that the programmable unijunction transistor Q5 will not turn on again to cause the blower rate 50 to stutter. Thus, in the embodiment of the invention illustrated, the programmable unijunction transistor Q5 controls the triac Q9. Pulsation of the killer programmable unijunction transistor Q6 stops Q5 from pulsing and therefore stops the triac Q9 from turning on the blower.

Referring to FIGS. 2 through 8, FIG. 2 illustrates the line voltage as supplied to the system 40 and to the motor 50 when the motor 50 is operating at full speed. FIG. 3 illustrates the voltage supplied to the control circuit. FIG. 4 is illustrative of the voltage at the capacitor C9 with the motor 50 off, while FIG. 5 is illustrative of the voltage at the capacitor C9 with the motor running slowly. FIG. 6 illustrates the voltage applied to the motor 50 when the motor 50 is running slowly while FIG. 7 illustrates the voltage at the capacitor C9 with the motor 50 operating at approximately 90 percent full speed and FIG. 8 illustrates the voltage applied to .the motor'50 at approximately 90 percent full speed.

While a preferred embodiment of the invention has been illustrated and described, it will be understood that various changes and modifications may be made without departing from the spirit of the invention.

What is claimed is:

1. In a speed control system adapted to vary the speed of an electric motor as a function of time, the combination including solid state means energizable after a predetermined time delay to start said motor at a low speed and to gradually increase the speed of said motor until full motor speed is reached, said system in cluding means operable to delay the start of said motor, means operable to maintain said motor at full speed and means operable to decrease the speed of said motor gradually to a stopped condition, said solid state means including a Zener diode and a timing capacitor, said Zener diode being effective to increase the speed of said motor by charging said timing capacitor.

2. In a speed control system adapted to vary the speed of an electric motor as a function of time, the combination including solid state means energizable after a predetermined time delay to 'start said motor at a low speed and to gradually increase the speed of said motor until full motor speed is reached, said system including means operable to delay the start of said motor, means operable to maintain said motor at full speed, and means operable to decrease the speed of said motor gradually to a stopped condition, said system including means comprising a pair of programmable unijunction transistors energizable to stop said motor to a speed greater than the startingspeed of said motor.

3. The combination as set forth in claim 2 including means for adjusting the starting speed of said motor.

4. The combination as set forth in claim 2 including means for adjusting the stopping speed of said motor.

5. A furnace blower speed control system adapted to vary the speed of a furnace blower motor as a function of time and operable in conjunction with a furnace burner, said system comprising, in combination, solid state means energizable to start said motor at av low speed at a predetermined time after said burner ignites and togradually increase the speed of said motor until full motor speed is reached, and means for gradually decreasing the speed of said motor commencing at a predetermined time after said burner is extinguished,

said solid state means including a triac, a programmable unijunction transistor, a Zener diode and a timing capacitor, said programmable unijunction' transistor controlling the energization of said triac, said Zener diode controlling the charging of said timing capacitor.

6. The combination asset forth in claim 5 wherein said system includes means comprising a pair of programmable unijunction transistors energizable to stop said motor at a speed greater than the starting speed of said motor.

7. The combination as set forth in claim 6 including meansfor adjusting the starting speed of said motor.

8. The combination as set forth in claim 6 including means for adjusting the stopping speed of said motor.

9. A furnace blower speed control system adapted to vary the speed of a furnace blower motor as a function of time and operable in conjunction with an ignitable furnace burner, said system comprising, in combination, solid state means energizable to start said motor at a relatively low speed after a predetermined time following ignition of said burner, means in said system for gradually increasing the speed of said motor until full motor speed is reached, and means operable to maintain said motor at full speed for a predetermined time after said burner is extinguished and thereafter to gradually decrease the speed of said motor to a stopped condition, said .solid state means including a triac, a programmable unijunction transistor, a Zener diode and a timing capacitor, said programmable unijunction transistor controlling the energization of said triac, said Zener diode controlling the charging of said timing capacitor, means comprising a pair of programmable unijunction transistors energizable to stop said motor at a speed greater than the starting speed of said motor, and means for adjusting the stopping speed of said motor.

10. In a fumace blower speed control system adapted to vary the speed of a fumace blower motor as a function of time and operable in conjunction with an ignitable fumace burner adapted to be connected to a main line source of AC current, a low voltage control circuit including motor control means energizable to start said motor at a low speed in response to ignition of said burner and to gradually increase the speed of said motor until full motor speed is reached, means in said circuit for maintaining said motor at full speed while said burner is ignited, and means in said circuit for gradually decreasing the speed of said motor to a stopped condition after said burner is extinguished.

11. The combination as set forth in claim 10 including means effective to delay the energization of said motor control means for a predetermined time following ignition of said burner.

12. The combination as set forth in claim 10 including means effective' to delay the deenergization of said motor for a predetermined time following extinguishment of said burner.

13. The combination as set forth in claim 10 including meanseffective to delay the energization of said motor control means for a predetermined time following ignition of said burner, and means effective to delay the deenergization of said motor control means for a predetermined time following extinguishment of said burner.

14. The combination as set forth in claim 10 wherein said motor control means includes a triac and a timing capacitor, means including a Zener diodeeffective to gradually increase the speed of said motor by charging said timing capacitor, a pair of programmable unijunction transistors energizable to stop said motor at a speed greater than the starting speed of said motor, means for adjusting the starting speed of said motor, and means for adjusting the stopping speed of said mo- 

1. In a speed control system adapted to vary the speed of an electric motor as a function of time, the combination including solid state means energizable after a predetermined time delay to start said motor at a low speed and to gradually increase the speed of said motor until full motor speed is reached, said system including means operable to delay the start of said motor, means operable to maintain said motor at full speed and means operable to decrease the speed of said motor gradually to a stopped condition, said solid state means including a Zener diode and a timing capacitor, said Zener diode being effective to increase the speed of said motor by charging said timing capacitor.
 2. In a speed control system adapted to vary the speed of an electric motor as a function of time, the combination including solid state means energizable after a predetermined time delay to start said motor at a low speed and to gradually increase the speed of said motor until full motor speed is reached, said system including means operable to delay the start of said motor, means operable to maintain said motor at full speed, and means operable to decrease the speed of said motor gradually to a stopped condition, said system including means comprising a pair of programmable unijunction transistors energizable to stop said motor to a speed greater than the starting speed of said motor.
 3. The combination as set forth in claim 2 including means for adjusting the starting speed of said motor.
 4. The combination as set forth in claim 2 including means for adjusting the stopping speed of said motor.
 5. A furnace blower speed control system adapted to vary the speed of a furnace blower motor as a function of time and operable in conjunction with a furnace burner, said system comprising, in combination, solid state means energizable to start said motor at a low speed at a predetermined time after said burner ignites and to gradually increase the speed of said motor until full motor speed is reached, and means for gradually decreasing the speed of said motor commencing at a predetermined time after said burner is extinguished, said solid state means including a triac, a programmable unijunction transistor, a Zener diode and a timing capacitor, said programmable unijunction transistor controlling the energization of said triac, said Zener diode controlling the charging of said timing capacitor.
 6. The combination as set forth in claim 5 wherein said system includes means comprising a pair of programmable unijunction transistors energizable to stop said motor at a speed greater than the starting speed of said motor.
 7. The combination as set forth in claim 6 including means for adjusting the starting speed of said motor.
 8. The combination as set forth in claim 6 including means for adjusting the stopping speed of said motor.
 9. A furnace blower speed control system adapted to vary the speed of a furnace blower motor as a function of time and operable in conjunction with an ignitable furnace burner, said system comprising, in combination, solid state means energizable to start said motor at a relatively low speed after a predetermined time following ignition of said burner, means in said system for gradually increasing the speed of said motor until full motor speed is reached, and means operable to maintain said motor at full speed for a predetermined time after said burner is extinguished and thereafter to gradually decrease the speed of said motor to a stopped condition, said solid state means including a triac, a programmable unijunction transistor, a Zener diode and a timing capacitor, said programmable unijunction transistor controlling the energization of said triac, said Zener diode controlling the charging of said timing capacitor, means comprising a pair of programmable unijunction transistors energizable to stop said motor at a speed greater than the starting speed of said motor, and means for adjusting the stopping speed of said motor.
 10. In a furnace blower speed control system adapted to vary the speed of a furnace blower motor as a function of time and operable in conjunction with an ignitable furnace burner adapted to be connected to a main line source of AC current, a low voltage control circuit including motor control means energizable to start said motor at a low speed in response to ignition of said burner and to gradually increase the speed of said motor until full motor speed is reached, means in said circuit for maintaining said motor at full speed while said burner is ignited, and means in said circuit for gradually decreasing the speed of said motor to a stopped condition after said burner is extinguished.
 11. The combination as set forth in claim 10 including means effective to delay the energization of said motor control means for a predetermined time following ignition of said burner.
 12. The combination as set forth in claim 10 including means effective to delay the deenergization of said motor for a predetermined time following extinguishment of said burner.
 13. The combination as set forth in claim 10 including means effective to delay the energization of said motor control means for a predetermined time following ignition of said burner, and means effective to delay the deenergization of said motor control means for a predetermined time following extinguishment of said burner.
 14. The combination as set forth in claim 10 wherein said motor control means includes a triac and a timing capacitor, means including a Zener diode effective to gradually increase the speed of said motor by charging said timing capacitor, a pair of programmable unijunction transistors energizable to stop said motor at a speed greater than the starting speed of said motor, means for adjusting the starting speed of said motor, and means for adjusting the stopping speed of said motor. 